The primary objectives of the proposed research are: (1) To determine if heterogeneity in the requirements for immunoglobulin and complement for phagocytosis and intracellular killing by human polymorphonuclear leukocytes (PMNs) exists among strains of the same species of Bacteroides, as well as among strains of different species. (2) To determine if immunoglobulin is required for activation of the alternative complement pathway by the Bacteroides strains or for other steps in the opsonic process. (3) To determine the species specificity of natural and immune IgG and IgM opsonizing antibodies to B. fragilis and B. thetaiotaomicron and the moieties of the outer membrane to which the antibodies are directed. (4) To determine the moieties of the outer membrane of B. fragilis and B. thetaiotaomicron which are responsible for activation of the alternative complement pathway. The primary methods to be used are: (1) Assays for measuring phagocytosis and intracellular killing of bacteria by leukocytes, and for measuring C3 to C9 consumption. (2) Chromatographic methods for purification of IgG, IgM, components of the outer membrane, and depletion of IgG from serum. (3) Folin method for quantitating protein in the IgG and IgM preparations and in components of the outer membrane. (4) Scintillation counting for the identification of carbohydrate in components of the outer membrane.